In making lithographic printing plates, a step of coating a desensitizing gum, called a gumming-up step, is provided as a final step for protecting non-image areas (areas which retain water to repel a printing ink).
The desensitizing gum is applied to non-image areas to protect the hydrophilicity of the non-image areas as well as to protect the areas from being stained or flawed by adhesion of fingerprints, fats and oils, dusts, etc. upon correction of image areas such as retouching or deletion, during storage before printing and after plate making or storage before reuse, or upon handling to mount the printing plate on a press and, in addition, to prevent oxidative stains. Known gum compositions for lithographic printing plates which include compositions comprising an aqueous solution of gum arabic, cellulose gum or a water-soluble high molecular substance containing carboxy groups in the molecule and optionally containing a pH-adusting agent, an antiseptic, etc, have been popularly used. However, these conventionally known compositions have the following problems. That is, in the final step of finishing a printing plate, a gum solution is applied to the printing plate and spread all over the plate surface a using sponge or a cotton pad, followed by polishing the plate surface with a cotton pad or a cloth wiper until it becomes dry, upon which the water-soluble high molecular substance is thickly coated in part on image areas (areas which receive an ink). The thickly coated image areas have such a poor ink receptivity in printing that many copies must be printed before the image fully accepts ink. This phenomenon is generally called image blinding (so-called blinding). Where the above-described phenomenon takes place, the plate generally must be subjected to a washing step with water or weakly acidic solution to thereby remove the hydrophilic colloid adsorbed on the image areas for reproducing image areas. This washing step consumes much time, and hence there has been developed a removing solution for desensitizing gum as described in U.S. Pat. No. 4,024,085.
The coating of image areas with fats and oils before the gumming-up step has been carried out for the purpose of protecting ink-receptive properties of the image areas. However, this makes the plate-making step complicated and deteriorates workability and, in addition, it is not preferable due to the pollution and health hazard problems. Accordingly, attempts have been made at using a water-soluble organic high molecular compound which does not causing image blinding as a desensitizing gum. For example, U.S. Pat. No. 4,095,525, and British Pat. No. 2,010,298, West German Pat. No. 2,504,594, and Soviet Pat. No. 623,755 disclose dextrin, pullulan and its derivatives, carboxy-containing polyacrylamide derivatives, metyl acrylate (or methacrylate) grafted polyacrylamide copolymor, etc. However, these compounds are not desirable because they exert only a poor desensitizing action on non-image areas.